This invention relates to apparatus for the continuous dewatering of aqueous suspensions such as slurries and sludges resulting from the purification of waste water.
Recently, filter belts running over a number of guide rolls similar to those employed in paper production have been employed in sludge dewatering systems. Such belts pass through two different zones; a feeding and initial partial dewatering zone and a main dewatering zone. At the initial dewatering zone, the aqueous liquid passing through the filter medium leaves a more or less solid filter cake on the belt which is further dewatered by compression and/or a wicking action whereby the final product has a higher content of solids and a reduced residual moisture level.
Heretofore, difficulties have been encountered in dewatering the slurry sufficiently in the initial partial dewatering zone to form a firm layer of solids which will not give way under the pressure exerted thereon in the main dewatering zone. One attempt to solve this problem has been to extend the initial dewatering zone so that its length equals that of the main dewatering zone. However, this requires an increase in space and is expensive. Another attempt to solve the problem has been to make the unit more compact by positioning one dewatering zone over another so that initial dewatering takes place on the upper flight of an upper, endless belt. The partially dewatered sludge then passes into the main dewatering zone where it is pressed between the lower flight of the upper endless belt and a main filter belt to form a filter cake layer. While this apparatus is compact it offers no solution as far as the cost of equipment is concerned.
Another form of prior art dewatering apparatus consists of two zones arranged in alignment with each other with the feeding and initial dewatering zone of the belt rising sharply and forming one wall of a slurry collecting tank. The front wall of the collecting tank has an inlet for introducing the suspension. While the long retention time in the collecting tank and the increased hydrostatic head prevailing therein will increase the dewatering efficiency of the initial dewatering zone, difficulties are encountered in providing an effective seal where the outer surfaces of the belt move adjacent the walls of the collecting tank. That is, to provide an effective seal, the belt is susceptible to excessive wear whereby it has only a short service life.